Over the two-day hearing, the union argued there was no evidence of interference with the ballot and that the “legitimate partisan campaigning” for the “yes vote” was in line with the rules.
However, Mr Justice Swift, who chaired the hearing, said that strike action risked the “delay or non-delivery” of electoral material produced by political parties and of postal votes.
Citing the 2017 general election, in which 8.4 million votes were cast via post, he added that he attached “particular importance” to the timely deliverance and receipt of all ballot papers before December 12.
Mr Swift said: “In this case, in respect of the conduct of the General Election and the part played in that election by postal votes, there is a relevant wider public interest that is material to my conclusion that an injunction should be granted.”
Shane O’Riordain, Royal Mail managing director of regulation and corporate affairs, told The Telegraph: “It’s important that we are able to effectively deliver postal votes, especially as rising numbers of people are choosing to cast their ballots through the post. But we strongly believe there is no case for industrial action in the first place.”
A source at the CWU said the union would still attempt to strike during the election if they win an appeal against the injunction. If the appeal fails, however, the 110,000 workers will be re-balloted in the New Year.